Agriculture Reference
In-Depth Information
system is greatly impaired. Roots tend to spread along the surface of the hard pan layer. Better
root growth with wider or uniform distribution can improve nutrient uptake and use efficiency
by crop plants.
8.2.1.4 Improving Soil Structure
The development of well-structured soils is one of the major objectives for achieving sustainable
agricultural systems (Diaz-Zorita et al., 2004; Bhattacharyya et al., 2013). The clustering of soil
particles (sand, silt, and clay) into aggregates or peds, and their arrangement into various patterns,
is termed soil structure. Soil structure is a physical manifestation of the processes involved in the
development of soil bodies. It is one of the differentiating characteristics among soils that it is
always included in the morphological description of soil profiles. From the agronomic standpoint,
soil structure affects plant growth through its influence on infiltration, percolation, and retention
of water, soil aeration, and mechanical impedance to root growth. Six et al. (2004) reported that
important aspects for optimal functioning of soils that would deliver services for agriculture and
reduce soil erosion are soil structure, a property largely defined by aggregation. It follows that well-
aggregated soil is critical for nutrient and water use efficiency. Aggregates provide porosity, which
not only improves water retention and the recovery of nutrients and water for crops but also enables
better root growth.
Soil structure influences crop productivity because it is related to soil organic matter content
(Bird et al., 2000), microbial biomass (Franzluebbers et al., 1996), N mineralization (Hassink,
1994), and crop N requirement (Oberle and Keeney, 1990). For example, Oberle and Keeney (1990)
reported that corn N requirements were lower on irrigated loamy sandy soils than on silt loam soils
in a study conducted in the northern U.S. corn belt. The contribution of soil texture to crop yield
variability across the landscape has been reported by many workers (McConkey et al., 1996; Cox
et al., 2003; Cambouris et al., 2006; Nyiraneza et al., 2012), indicating the need for variable-rate N
fertilization (Nyiraneza et al., 2012). Other studies, such as those conducted in eastern Canada by
Nolin et al. (1989) and Leclerc et al. (2001), have demonstrated that soil texture is the most impor-
tant component in the soil fertility classification system.
Soil compaction is a serious soil structural problem on many fine-textured soils. Compaction
of soil affects nearly all soil properties and functions, which in turn affect the growth and pro-
ductivity of plants (Gregorich et al., 2011). Compacted soils are characterized by high strength,
high bulk density, and low hydraulic conductivity and air-filled porosity (Lowery and Schuler,
1991; Blanco-Canqui and Lal, 2008). Soil with high strength impedes the growth (Montagu
et al., 2001; Bengough et al., 2006), distribution (Kasper et al., 1991), and function (Tardieu,
1994) of roots. With these restrictions, root systems develop superficially and as a consequence,
the roots explore a smaller volume of soil and hence intercept a limited amount of water and
nutrients (Oussible et al., 1992). Lower matric hydraulic conductivity and porosity can restrict
soil gas diffusion and water availability. Reduced O
2
content in compacted surface soils, result-
ing in part from reduced porosity and soil structure degradation (Topp et al., 2000), can in turn
affect the transport, absorption, and transformation (e.g., mineralization) of nutrients (Lipies
and Stepniewski, 1995). Gregorich et al. (2011) reported that compaction degrades the soil struc-
ture and consequently reduces N uptake by corn. As a consequence, relatively large amounts of
postharvest soil N in compacted soils increase the risk of N loss to the environment by denitri-
fication or leaching.
Soil structure can be improved with the addition of organic manures and appropriate crop rota-
tion. The use of fertilizers in adequate amount for crop production has a positive effect on crop
residue (Campbell et al., 1993a); in turn, this will have a direct effect on soil aggregation (Campbell
et al., 1995). The latter will influence soil structure, erodibility, soil workability, and water infiltra-
tion. Hence, the adoption of proper fertilization practice is imperative for maintaining the soil in
good physical condition for crop production. Legumes (green manure and particularly grass-alfalfa
hay crops) are very effective in promoting good soil aggregation (Birch, 1959).
